Although a lithium ion secondary battery has a high output density and capacity density, electrolyte may be decomposed by the heat generated in an abnormal situation such as short circuiting and overcharging in some cases because of the use of an organic solvent therein and fire may occur in the worst case. In order to prevent such a case, several safety functions are incorporated in a lithium ion secondary battery, including the shutdown function of a separator. The shutdown function is a function of blocking micro pores of a separator due to thermal fusion or the like in an abnormal heat generation in a battery so as to stop the electrochemical reaction from proceeding through inhibition of ionic conduction in the electrolyte. In general, it is believed that higher safety is obtained as the shutdown temperature decreases. Examples of the reasons for using polyethylene as the component of a separator include having a proper shutdown temperature. In the case of a battery having high energy, however, the temperature in the battery continues to rise even after the electrochemical reaction is stopped from proceeding by shutdown, so that a problem of short-circuiting both ends occurs due to rapture of the membrane of a separator, or the like.
On the other hand, applications of nonaqueous electrolyte batteries such as a lithium ion secondary battery to the usage in need of charging/discharging a large current in a short time, for example, the usage in electric vehicles and hybrid electric vehicles, are rapidly expanding. Accordingly, the safety characteristics are required to be improved without sacrificing output characteristics severely required for the usage.
In Patent Literature 1, a multilayer porous membrane including a porous membrane including a polyolefin resin as a main component, and a porous layer having a mass fraction of inorganic filler of 50% or more and less than 100% laminated on at least one side of the porous membrane is disclosed. In the literature, a technique for achieving both of an excellent heat resistance for preventing both ends from being short-circuited and an excellent shutdown function, even in the case of having a large amount of heat generated in an abnormal heat generation.